Thursday, 9 June 2016

Finally... A High Altitude Balloon flight with Raspberry Pi

It had been a long
journey from last year's Skycademy to get to this launch. Being situated just a few miles from
Heathrow was always going to mean that launching from school would be
tricky unless it involved everybody getting up a silly-o'clock
(before flights started) and chasing through rush hour.

So finding a launch
site close enough to be practical but far enough outside the LondonControl Zone (which defines the busy airspace around London) to gain
CAA approval took a several months and a few applications and phone
calls.

But now I had one I
thought was suitable;. Even then, our launch permission comes with
wind restrictions: the balloon must drift South or South West (i.e.
away from London). This isn't as bad as it sounds because,
realistically, you wouldn't want to chase a balloon heading in any
other direction.

Thursday's flight
was to be the first, long-awaited test from this site. Wind
conditions over Easter were rubbish and we'd already aborted a
planned launch on Tuesday because of bad weather (heavy ran and
wind). Although conditions had improved and it was a dry day, it
remained overcast and gusty (30mph) during the morning. Although
things looked like improving even further by the weekend, Nick
wouldn't be able to make it then so I decided to press ahead with the
launch. Despite the blowy ground winds, the balloon wasn't due to
travel too far and the predicted landing site was in the South Downs
National Park. Tweaking the launch parameters to avoid coming down
in Petersfield meant the burst altitude was lower than we'd like at
just(!) 27km, but as this was really a test of the launch site
conditions, we didn't mind too much.

We arrived at the
site at about 10:30 and set about finding a sheltered area. When I'd
reconnoitred a couple of days earlier I'd found a ridge that seemed
to provide a block for the wind. Although it was deficiently calmer
there, it was still pretty gusty and an ordinary balloon staked out
on the ground sheet was being constantly blown at about a 45 degree
angle. However the sun was started to break up the clouds and with a
long, clear area free from obstructions in the balloon's direction of
travel, we decided to go ahead, over-filling if necessary.

Apart from getting
slightly lost on the way back to the car to retrieve all the kit,
pre-flight prep went smoothly. We'd tested everything in the days
before the flight and our payload appeared on the HabHub map a minute
after we booted up the Raspberry Pi A+. We were lucky enough to have
two RTTY receivers (with an SDR dongle as back-up) and a LORA
receiver with us (plus another one connected to a roof aerial back at
my house) so we were fairly confident of our ability to track the
balloon.

We attached the
parachute to the payload and got ready to fill the balloon. As soon
as it began to inflate it was clear that adding enough helium for the
correct neck lift was going to be tricky to estimate: the balloon was
being buffeted by the wind and required all four of us to hold
steady. Rather than under-filling, (we think) we put in more helium
than required. Certainly the balloon was pulling away quite fiercely
by the end of the process.

Now to launch. The
chirping from the RTTY receiver told us that the radios were still
working and after a quick check to see that images were being
uploaded, we were ready to go.

Picture by @duck_star

Ozzy held the
balloon while Jasper had the parachute and I had the payload. The
balloon was released and Oz gently played out the line until it
reached the parachute. Jasper repeated the procedure until the cord
started to tug on the payload and then I let go. Smooth!

Everything soared up
into the sky, rapidly gaining height within a few seconds. By the
time we'd picked up cameras to take photos, it was nearly out of
site.

The Pi's eye view just after launching

A quick but thorough
pack-up and we headed back to the car park (managing to go the long
way round again). Mag-mount aerials deployed on the roof of the car
and we were off. The original predictions had the balloon coming down
just west of Petersfield, but the in-flight prediction was now
showing a touch-down nearer to Winchester so we headed there.

Pi's eye view 15 minutes into the flight

My roof-mounted
aerial back at home picked up the LORA transmissions from about 700m
altitude which was impressive and we soon saw several other RTTY
trackers uploading data to the website too.

By the time we
arrived in Winchester, the balloon seemed to have been caught in a
mini-tornado, spiralling round on itself over Liphook and never quite
managing to break free and head off towards us despite steadily
gaining altitude. Parking up on the roof of a multi-story car park we
had lunch and decided that the balloon was never going to make it
this far west before bursting: because we reckoned we'd added more
helium than required, we were expecting the burst to come earlier
than at the 27km we'd initially been aiming for.

As we headed back
East the balloon finally escaped its looping trajectory and started
heading west, gaining altitude even more rapidly. This also caused
the predicted landing area to shift towards Alresford, so we drove
there and found a nice pub with a beer garden to wait and see what
happened. By this point the altitude was approaching 26km so we were
expecting a burst at any time. But the altitude kept increasing until
we we approaching 30km. Surely it must burst now?

Pi's eye view from 34km up

We continued to
watch and fiddle with our receivers (Ozzy somehow managed to tune his
Yupiteru into a music station), chasers of all ages becoming
increasingly excited. The altitude continued to rise:
31km...32km...33km...34km… until at nearly 35K – and with much
shouting and excitement from the younger team members – the HabHub
icon changed from a balloon to a parachute and the altitude dropped
rapidly.

Although the landing
zone predictions were fluctuating, they seemed to be staying in
roughly the same area so we made for Cheriton. 3/4G connectivity was
quite patchy in this region so we kept loosing web access – and our
ability to upload telemetry from our receivers. Getting confused
(and slightly lost) we pulled up in a handy lay-by in a country lane
where we had a strong 4G signal. We looked at where we were and where
the payload was predicted to land and were pleased to discover that
the two locations were almost the same. Surrounded by open fields
this seemed like an perfect spot and we got out the binoculars and
stood around in the sunshine hoping that we'd actually be able to see
the payload drift in on its parachute.

It was not to be.

Suddenly at about
1km up, the predicted landing spot suddenly changed. Maybe the gusty
winds just blew it off course but the new touchdown spot was a couple
of miles away, right in the centre of (the excellently named) Itchen
Abbas. This was less than ideal. Plenty of trees, roofs, private
gardens and even a river to potentially foil our safe recovery plans.

We parked up at the
village hall and checked our data. Annoyingly, despite having has 5
bars of 4G in a remote field two miles away, here among the houses
and pubs of a fairly large village we had almost no signal on any
network. The good news was that we were still getting telemetry from
the payload and it appeared to stationary and on the ground. Typing
the latitude and longitude into Google Maps and managing to find a
scrap of 3G, we set off on foot.

Pi's eye view on the way down

After a few minutes
of heading in the wrong directions we got ourselves oriented and
started to close the gap between us and the payload. As we rounded a
corner and looked at the tablet I realised that the coordinates put
the landing site in the churchyard. We rushed in and there is was,
sitting peacefully amongst the weathered gravestones. It had landed
with the GPS facing upwards and with no signs of damage. There was
very little of the balloon left, suggesting that it had been a violent
burst when it eventually happened.

After some
respectful celebrations (we were in a churchyard after all) and the
obligatory recovery photos, we switched off the Pi and headed back to
the car for the journey home.

On reflection, the
flight was a great success. After the launch itself I was just happy
that we'd managed to sort out all the logistics and permissions, and
actually get a balloon off the ground. But having a successful chase
and recovery, coupled with the amazing altitude our payload achieved,
it was a really great day. All the tech worked and the only mishap
was me forgetting to turn off the valve on the helium canister before
removing the flexi-filler attachment thus wasting a load of gas.

We even got back
home in time for tea.

Take-aways from me:

The boys really
enjoyed the day. Some of the questions they asked demonstrated that
(a) they hadn't really been listening when I'd explained some aspects
of the flight and science as we prepared in the days prior to launch,
and (b) actually being involved in physical filling of the balloon,
the technical tracking and watching the live telemetry really
captivated them and got them thinking and asking questions in a way
that simply being told about stuff never will.

This is really a
team sport. Four people seemed the minimum needed to make things run
smoothly. Any less and we'd have had to make multiple trips from the
car to the launch site and would have really struggled with the
filling in the windy conditions. Multiple eyes on multiple screens
also made tracking and navigating easier.

Having a fixed
position LORA receiver with a reliable Internet connection is a good
idea. It means that you have telemetry being uploaded even when you
have no reception in the chase car, or are unable to upload yourself
due to no local 3G/4G.

Altitude telemetry from LORA

A checklist
including pictures of how to fill and tie off the balloon is really
useful. Diagrams and printouts of calculations save you having to
remember stuff when you're busy.

You can never have
too many receivers and Internet uplinks (preferably on different
networks) with you.

Picture by @duck_star

Don't rush into a
launch if conditions aren't suitable. But equally, don't be put off
if absolutely everything isn't ideal. It probably never will be, and
predictions will never be perfectly accurate either.